Website Photodoto has a nice article I just found about the how and why of making great portrait photos. One of the passages read-
"I find that the best place to start is with the eyes. It’s natural to meet someone’s gaze even when reproduced on a 2-dimensional medium like a computer screen or film. The eyes can draw the viewer in, acting as a focal point for the entire image. This is why it is usually critical that the eyes are in focus. I can’t recall seeing a portrait that I really liked where the eyes were not in focus. And this applies whether you are taking photos of people, pets, or animals in the wild—focus on the eyes."
(of course, the shower curtain in the background is optional)
Read Making Portraits